cmd/internal/dwarf: use []*Var instead of linked lists

Passes toolstash -cmp.

Change-Id: I202b29495ca1aaf3c52879fa99fdc0a4b86703af
Reviewed-on: https://go-review.googlesource.com/36419
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Josh Bleecher Snyder <josharian@gmail.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/cmd/internal/dwarf/dwarf.go

@@ -25,7 +25,6 @@ type Var struct {
 	Abbrev int // Either DW_ABRV_AUTO or DW_ABRV_PARAM
 	Offset int32
 	Type   Sym
-	Link   *Var
 }
 
 // A Context specifies how to add data to a Sym.
@@ -565,7 +564,7 @@ func HasChildren(die *DWDie) bool {
 
 // PutFunc writes a DIE for a function to s.
 // It also writes child DIEs for each variable in vars.
-func PutFunc(ctxt Context, s Sym, name string, external bool, startPC Sym, size int64, vars *Var) {
+func PutFunc(ctxt Context, s Sym, name string, external bool, startPC Sym, size int64, vars []*Var) {
 	Uleb128put(ctxt, s, DW_ABRV_FUNCTION)
 	putattr(ctxt, s, DW_ABRV_FUNCTION, DW_FORM_string, DW_CLS_STRING, int64(len(name)), name)
 	putattr(ctxt, s, DW_ABRV_FUNCTION, DW_FORM_addr, DW_CLS_ADDRESS, 0, startPC)
@@ -576,7 +575,7 @@ func PutFunc(ctxt Context, s Sym, name string, external bool, startPC Sym, size
 	}
 	putattr(ctxt, s, DW_ABRV_FUNCTION, DW_FORM_flag, DW_CLS_FLAG, ev, 0)
 	names := make(map[string]bool)
-	for v := vars; v != nil; v = v.Link {
+	for _, v := range vars {
 		if strings.Contains(v.Name, ".autotmp_") {
 			continue
 		}
@@ -602,3 +601,11 @@ func PutFunc(ctxt Context, s Sym, name string, external bool, startPC Sym, size
 	}
 	Uleb128put(ctxt, s, 0)
 }
+
+// VarsByOffset attaches the methods of sort.Interface to []*Var,
+// sorting in increasing Offset.
+type VarsByOffset []*Var
+
+func (s VarsByOffset) Len() int           { return len(s) }
+func (s VarsByOffset) Less(i, j int) bool { return s[i].Offset < s[j].Offset }
+func (s VarsByOffset) Swap(i, j int)      { s[i], s[j] = s[j], s[i] }

src/cmd/internal/obj/objfile.go

@@ -561,7 +561,7 @@ func gendwarf(ctxt *Link, text []*LSym) []*LSym {
 		dw = append(dw, dsym)
 		dsym.Type = SDWARFINFO
 		dsym.Set(AttrDuplicateOK, s.DuplicateOK())
-		var vars dwarf.Var
+		var vars []*dwarf.Var
 		var abbrev int
 		var offs int32
 		for a := s.Autom; a != nil; a = a.Link {
@@ -583,23 +583,26 @@ func gendwarf(ctxt *Link, text []*LSym) []*LSym {
 			default:
 				continue
 			}
+
 			typename := dwarf.InfoPrefix + a.Gotype.Name[len("type."):]
-			dwvar := &dwarf.Var{
+			vars = append(vars, &dwarf.Var{
 				Name:   a.Asym.Name,
 				Abbrev: abbrev,
-				Offset: int32(offs),
+				Offset: offs,
 				Type:   Linklookup(ctxt, typename, 0),
+			})
 		}
-			dws := &vars.Link
-			for ; *dws != nil; dws = &(*dws).Link {
-				if offs <= (*dws).Offset {
-					break
-				}
-			}
-			dwvar.Link = *dws
-			*dws = dwvar
+
+		// We want to sort variables by offset, breaking ties
+		// with declaration order. Autom holds variables in
+		// reverse declaration order, so we reverse the
+		// assembled slice and then apply a stable sort.
+		for i, j := 0, len(vars)-1; i < j; i, j = i+1, j-1 {
+			vars[i], vars[j] = vars[j], vars[i]
 		}
-		dwarf.PutFunc(dctxt, dsym, s.Name, s.Version == 0, s, s.Size, vars.Link)
+		sort.Stable(dwarf.VarsByOffset(vars))
+
+		dwarf.PutFunc(dctxt, dsym, s.Name, s.Version == 0, s, s.Size, vars)
 	}
 	return dw
 }